Capacitors for use in electronic instruments such as potable telephone and personal computer are demanded to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferred because of its large capacitance for the size and good performance. In this tantalum capacitor, a sintered body of tantalum powder is generally used for the anode moiety. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the weight of the sintered body or to use a sintered body increased in the surface area by pulverizing the tantalum powder.
The former method of increasing the weight of the sintered body necessarily involves enlargement of the capacitor shape and cannot satisfy the requirement for downsizing. On the other hand, in the latter method of pulverizing tantalum powder to increase the surface area, the pore size of the tantalum sintered body decreases or closed pores increase at the stage of sintering and therefore, impregnation of the cathode agent in the later process becomes difficult. As one of means for solving these problems, a capacitor using a sintered body of powder of a material having a dielectric constant larger than that of tantalum is being studied. The materials known to have a larger dielectric constant include niobium.
However, capacitors using a sintered body manufactured from these materials suffer from insufficient high-temperature characteristics and cannot be used in practice. The ratio between the initial capacitance C0 at room temperature and the capacitance C after the capacitor is left standing in an atmosphere of 105° C. for 2,000 hours while applying a voltage and then returned to room temperature, that is, (C–C0)/C0 is defined as a high-temperature property. When a sintered body is electrolytically oxidized and then combined with another part electrode to produce a capacitor, insofar as tantalum powder is used, the high-temperature property usually falls within ±20%, but if a conventional niobium powder is used, some capacitors cannot have a high-temperature property falling within ±20%.
As such, capacitors using a niobium powder must be estimated low in the reliability at room temperature and because of this, these capacitors are judged defective in the service life and not used in practice.
As for the technology related to the present invention, a capacitor using partially reduced niobium oxide obtained by heat-treating diniobium pentoxide (Nb2O5) in the presence of hydrogen, which has a large capacitance and excellent leakage current characteristics, has been proposed (see, WO 00/15555). However, when a capacitor is manufactured using niobium oxide obtained in a test by the present inventors and subjected to an accelerated test at a high temperature, the capacitor performance is deteriorated and not satisfied.